1. Field of the Invention
The present invention relates to an intermediate-frequency processing circuit suitable for processing an intermediate frequency in different television transmission systems such as the L system and the B/G system.
2. Description of the Related Art
In recent years, a variety of multitelevision receivers which can receive a plurality of television broadcast systems of different television signal transmission systems have been developed. There are a variety of television systems. For example, the L system is employed in France, and the B/G system is employed in West Germany, East Germany, Spain, and Italy. FIGS. 1A to 1C show relationships among picture intermediate frequency P, sound intermediate frequency S, and local frequency Lf in the VHF low-band, VHF high-band, and whole UHF band in the L system. FIGS. 2A to 2C show relationships among picture intermediate frequency P, sound itermediate frequency S, and local frequency Lf in the VHF low-band, VHF high-band, and whole UHF band in the B/G system. As can be seen from these figures, the difference between these television signal transmission systems is that the positional relationship between picture and sound intermediate frequencies P and S in the VHF low-band of the L system is reversed to that in the other cases. An FM-modulated sound signal is received in the B/G system, while an AM-modulated sound signal is received in the L system. A difference between the sound and video intermediate frequencies is 5.5 MHz in the B/G system, and is 6.5 MHz in the L system.
Conventionally, an intermediate-frequency (IF) processing circuit in a multitelevision receiver which can receive television signals of both the L and B/G systems comprises three surface acoustic wave (SAW) filters, and three IF processing integrated circuits (ICs). More specifically, the first SAW filter has pass-band characteristics such that, of signals output from a tuner, an IF signal of the B/G system and IF signals in the VHF high-band and UHF band of the L system are supplied to the first IF processing IC. The second SAW filter has pass-band characteristics such that, of signals output from the tuner, picture intermediate frequency P in the VHF low-band of the L system is supplied to the second IF processing IC. The third SAW filter has pass-band characteristics such that, of signals output from the tuner, sound intermediate frequency S in the VHF low-band of the L system is supplied to the third IF processing IC. The first IF processing IC is a video IC for demodulating picture and sound signals in the VHF high-band and UHF band of the L system. The second IF processing IC is a video IC for demodulating a picture signal in the VHF low-band of the L system. The third IF processing IC is a sound IC for demodulating a sound signal in the VHF low-band of the L system. In this manner, the conventional IF processing circuit has two video ICs and three (SAW) filters.
However, the video IC is constituted by an IF amplifier, a limiter amplifier, a video demodulator, a video amplifier, a sound demodulator, an IF automatic gain controller, an RF automatic gain controller, and an automatic fine tuning demodulator, and the like. Therefore, the circuit configuration is complicated and large. Thus, the number of components is increased, resulting in an increase in cost of the apparatus, degradation of reliability, poor maintenance properties, difficulty in design, and the like. Since the two video ICs are arranged, the DC levels of demodulation outputs of the two video ICs cannot be easily kept constant, and a DC reproducing circuit for adjusting the DC levels is required. In addition, since the two video ICs are arranged, operating points of the IF automatic gain controllers and the RF automatic gain controllers, and the output levels of the automatic fine tuning demodulators are shifted from each other.